Find the ISO 8601 week number and day of the year for any date.
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This calculator uses the ISO 8601 standard, where weeks start on Monday and week 1 of the year is defined as the week containing the year's first Thursday. This means the first few days of January can sometimes belong to the last week of the previous year, and the last few days of December can sometimes belong to week 1 of the following year.
ISO week numbers are widely used in business, manufacturing and project planning for consistent weekly reporting, since every ISO week has exactly 7 days and starts on the same weekday (Monday), unlike calendar weeks which can be split unevenly across months.
January 1, 2023 fell on a Sunday, and this calculator reports it as week 52 of ISO year 2022, not week 1 of 2023, since ISO week 1 is defined by the year's first Thursday, and that Sunday belonged to the same Monday-to-Sunday week as the final Thursday of December 2022. The very next day, January 2, 2023 (a Monday), correctly starts ISO week 1 of 2023.
Assuming January 1st is always in week 1, as the worked example shows, it depends entirely on which day of the week January 1st falls on, some years it belongs to the last week of the previous ISO year instead.
Mixing up the ISO week year with the calendar year near year boundaries, a date's 'ISO Week Year' result can differ from its calendar year for a handful of days at the very start or end of December/January.
Assuming this matches a US-style week numbering system, some US conventions start the week on Sunday and count week 1 differently, this calculator strictly follows the international ISO 8601 standard (Monday start, first-Thursday rule).
Near the start or end of a calendar year, a date's ISO week can belong to the adjacent year's week numbering, for example, December 31 might fall in week 1 of the following year, or January 1 might fall in the last week of the previous year.
This calculator follows the ISO 8601 standard, where weeks start on Monday. This differs from some calendars and regions that start the week on Sunday.
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